1. Technical Field
The present disclosure relates to optics. Specifically, the disclosure relates to systems and methods that involve the use of polarized light that is scattered from samples.
2. Description of the Related Art
Measurements of back-scattered light are of interest for several reasons. By way of example, remote sensing techniques, such as light detection and ranging (LIDAR), analyze back-scattered light. Back-scattered light also can provide information about the size distribution of particles of a scattering system. Specifically, photons traveling reversed paths constructively interfere in the back-scatter region. This leads to a peak in the exact back-scatter direction that is inversely proportional to the length of the path difference. Hence, the back-scattered light provides a measure of the spatial properties of the scattering system.
Spatial properties are used to define and describe parameters of a system. For example, a set of spatial properties for a system may include elements relating to location, change in location, speed, trajectory (straight or curved), orientation, relation of parts to the whole system, and spatial relations among adjacent objects. Examples of shape properties may include form of the shape, size, number of parts, form of the parts, and symmetry or asymmetry, and examples of surface properties could include color(hue), saturation or intensity, texture and temporal patterns (blinking or pulsating). Spatial frequency and amplitude are other spatial properties often used to define a system. An analogous example of a distribution of spatial frequencies are waves coming onto a shore. The waves can be really close together (having high spatial frequencies) or further apart (having lower spatial frequencies). Another property is the amplitude of these spatial frequencies, such as the height of the waves. Further, spatial properties such as spatial frequencies and amplitudes, the mean spatial frequency and its standard deviation, and the root mean square (rms) roughness of the system can be used to characterize properties such as a surface, particles in a cloud, or an aggregate of spores. Thus, the root mean square (rms) amplitude of the surface roughness of both waves may be the same, but the conditions of the waves are different. Specifically, the spatial frequencies of the cloud of particles may relate to the density of the cloud, where the larger the density of the cloud, the higher the spatial frequencies of the cloud particles. A further example is an aggregate of spores where the spatial frequency of the aggregate is dependent upon the size of the particles making up the aggregate. Example of the use of spatial frequencies use in analyzing systems are disclosed in U.S. Pat. Nos. 6,034,776 (Germer et al) and 6,042,998 (Brueck et al) and “Effect Of Refractive Index In Optical Particle Using Spatial Frequency Method”, H. H. Qui, W. Jia, Optical Communication 178 (2000) 199-210, all of which are fully incorporated herein.
In analyzing particle systems with light or optical means, measurement of light back-scattered from particles and surfaces can be difficult. For instance, some devices for measuring back-scatter require two measurements to be taken at different times. This is problematic when the scattering system is time dependent, such as when particles are passed through a flow cytometer.
Methods and apparatus for measuring or determining characteristics of particles, such as size and velocity, with optical means are disclosed in U.S. Pat. Nos. 4,154,529 (Dyott); 4,492,467 (Drain et. al.); 5,063,301 (Turkevich et. al.); 5,294,806 (Batchelder et. al.); 5,561,515 (Hairston et. al.); 5,627,642 ((Dhadwal et. al.); 5,999,257 (Myers et. al.); 6,587,200 (Riebel et. al.); 6,674,528 (Adachi et. al.); 6,704,105 (Swanson et. al.); 6,744,507 (Yamaguchi); 6,774,994 (Wyatt et. al.); and 6,778,271 (Watson et. al.), all of which are fully incorporated herein.